This invention relates to a magnetron, and more particularly to a low angle helically wound magnetron filament having a quadrilateral cross section which provides increased sag resistance at high operating temperatures.
Magnetrons are used as ultra-high frequency oscillators for use in microwave ovens or the like. Usually, in the case of industrial microwave ovens, the magnetron is capable of generating useful continuous radio frequency (rf) power in the range of 10 to 50 kilowatts (kw) at very high efficiency. A conventional magnetron, such as the RCA 8684 Large Power Magnetron, is useful as a 915-Mhz, 30 kw rf power source in industrial processing applications. In the operation of such a magnetron, the cathode filament operates at about 2000.degree. C. to provide 3 amperes of anode current. In order to achieve a power output of 50 kilowatts, it is necessary to operate a cathode filament at 2150.degree. C. to obtain 4 amperes of anode current. At such an elevated cathode filament temperature, the conventional helically wound circular cross-section filament tends to sag. Since a circular cross-section filament emits electrons over an arc of about 120.degree. , some of the electrons from the extremeties of the arc are turned by the magnetic field and return to the filament producing "back heating" which further increases the cathode filament temperature. In the aforementioned magnetron, the coil length of the wound filament is about 48.26 mm. The weight of the lengthy filament in conjunction with the high operating temperature further increases the tendency of the filament to sag. As the helically wound circular cross-section filament sags, it also rotates because of torque. These factors frequently result in a decrease in tube efficiency and premature tube failure. It is, therefore, desirable to provide a filament that can operate at a temperature of at least 2150.degree. C. without sagging.